Messages in this thread | | | Date | Fri, 13 Dec 2019 09:19:50 -0800 | From | Sean Christopherson <> | Subject | Re: [PATCH v5 2/2] kvm: Use huge pages for DAX-backed files |
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On Fri, Dec 13, 2019 at 03:07:31AM +0200, Liran Alon wrote: > > > On 12 Dec 2019, at 21:55, Barret Rhoden <brho@google.com> wrote: > > > >>>> Note that KVM already faulted in the page (or huge page) in the host's > >>>> page table, and we hold the KVM mmu spinlock. We grabbed that lock in > >>>> kvm_mmu_notifier_invalidate_range_end, before checking the mmu seq. > >>>> > >>>> Signed-off-by: Barret Rhoden <brho@google.com> > >>> > >>> I don’t think the right place to change for this functionality is > >>> transparent_hugepage_adjust() which is meant to handle PFNs that are > >>> mapped as part of a transparent huge-page. > >>> > >>> For example, this would prevent mapping DAX-backed file page as 1GB. As > >>> transparent_hugepage_adjust() only handles the case (level == > >>> PT_PAGE_TABLE_LEVEL).
Teaching thp_adjust() how to handle 1GB wouldn't be a bad thing. It's unlikely THP itself will support 1GB pages any time soon, but having the logic there wouldn't hurt anything.
> >>> As you are parsing the page-tables to discover the page-size the PFN is > >>> mapped in, I think you should instead modify kvm_host_page_size() to > >>> parse page-tables instead of rely on vma_kernel_pagesize() (Which relies > >>> on vma->vm_ops->pagesize()) in case of is_zone_device_page(). > >>> > >>> The main complication though of doing this is that at this point you > >>> don’t yet have the PFN that is retrieved by try_async_pf(). So maybe you > >>> should consider modifying the order of calls in tdp_page_fault() & > >>> FNAME(page_fault)(). > >>> > >>> -Liran > >> Or alternatively when thinking about it more, maybe just rename > >> transparent_hugepage_adjust() to not be specific to THP and better handle > >> the case of parsing page-tables changing mapping-level to 1GB. > >> That is probably easier and more elegant.
Agreed.
> > I can rename it to hugepage_adjust(), since it's not just THP anymore.
Or maybe allowed_hugepage_adjust()? To pair with disallowed_hugepage_adjust(), which adjusts KVM's page size in the opposite direction to avoid the iTLB multi-hit issue.
> > Sounds good. > > > > > I was a little hesitant to change the this to handle 1 GB pages with this > > patchset at first. I didn't want to break the non-DAX case stuff by doing > > so. > > Why would it affect non-DAX case? > Your patch should just make hugepage_adjust() to parse page-tables only in case is_zone_device_page(). Otherwise, page tables shouldn’t be parsed. > i.e. THP merged pages should still be detected by PageTransCompoundMap().
I think what Barret is saying is that teaching thp_adjust() how to do 1gb mappings would naturally affect the code path for THP pages. But I agree that it would be superficial.
> > Specifically, can a THP page be 1 GB, and if so, how can you tell? If you > > can't tell easily, I could walk the page table for all cases, instead of > > just zone_device().
No, THP doesn't currently support 1gb pages. Expliciting returning PMD_SIZE on PageTransCompoundMap() would be a good thing from a readability perspective.
> I prefer to walk page-tables only for is_zone_device_page(). > > > > > I'd also have to drop the "level == PT_PAGE_TABLE_LEVEL" check, I think, > > which would open this up to hugetlbfs pages (based on the comments). Is > > there any reason why that would be a bad idea?
No, the "level == PT_PAGE_TABLE_LEVEL" check is to filter out the case where KVM is already planning on using a large page, e.g. when the memory is backed by hugetlbs.
> KVM already supports mapping 1GB hugetlbfs pages. As level is set to > PUD-level by > tdp_page_fault()->mapping_level()->host_mapping_level()->kvm_host_page_size()->vma_kernel_pagesize(). > As VMA which is mmap of hugetlbfs sets vma->vm_ops to hugetlb_vm_ops() where > hugetlb_vm_op_pagesize() will return appropriate page-size. > > Specifically, I don’t think THP ever merges small pages to 1GB pages. I think > this is why transparent_hugepage_adjust() checks PageTransCompoundMap() only > in case level == PT_PAGE_TABLE_LEVEL. I think you should keep this check in > the case of !is_zone_device_page().
I would add 1gb support for DAX as a third patch in this series. To pave the way in patch 2/2, change it to replace "bool pfn_is_huge_mapped()" with "int host_pfn_mapping_level()", and maybe also renaming host_mapping_level() to host_vma_mapping_level() to avoid confusion.
Then allowed_hugepage_adjust() would look something like:
static void allowed_hugepage_adjust(struct kvm_vcpu *vcpu, gfn_t gfn, kvm_pfn_t *pfnp, int *levelp, int max_level) { kvm_pfn_t pfn = *pfnp; int level = *levelp; unsigned long mask;
if (is_error_noslot_pfn(pfn) || !kvm_is_reserved_pfn(pfn) || level == PT_PAGE_TABLE_LEVEL) return;
/* * mmu_notifier_retry() was successful and mmu_lock is held, so * the pmd/pud can't be split from under us. */ level = host_pfn_mapping_level(vcpu->kvm, gfn, pfn);
*levelp = level = min(level, max_level); mask = KVM_PAGES_PER_HPAGE(level) - 1; VM_BUG_ON((gfn & mask) != (pfn & mask)); *pfnp = pfn & ~mask; }
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